Such methods are required in the manufacture of silicon semiconductor devices including deposited layers of silicon which may contain doping inpurities. The deposited layers may be continuous with the crystal structure of a monocrystalline silicon substrate. The same applies when the semiconductor material to be deposited is silicon carbide (SiC).
In the case just mentioned, it is well known that the deposited layers must be very pure. There are several known methods for achieving these high levels of purity, one of which is cathode sputtering. In this process the material to be deposited consists of a solid target. A direct current electric field is generated in the vicinity of the target, in the presence of a plasma which may be generated by means of a high-frequency electric field. The direct current field causes ion bombardment of the target. This releases atoms of the target material, which are deposited on the nearby semiconductor substrate.
The quality of the deposit may be improved by electrically biasing the substrate relative to the plasma, for example by connecting the substrate to the target when the latter is one of the conductive surfaces between which the high-frequency field is formed, and this enables the direct current field to be obtained from the high-frequency source. In this case, the connection to the substrate is made via an impedance such as a variable capacitance (see CHRISTENSEN et al, "RF biasing through capacitive collector to target coupling in RF diode sputtering"--Journal of Physics E: Scientific Instruments, 1972, vol 5, United Kingdom). This known method is not used, however, when the deposit is obtained from a material in the gaseous phase, instead of from an expensive solid target.
Another known method consists in placing the substrate in a chamber through which is passed a gas from which the material to be deposited can be formed by thermal decomposition. This method is known as chemical vapour deposition (CVD). It requires that the substrate be heated, and has the disadvantage that the temperature required may result in deterioration of the substrate properties, for example if the substrate consists of a semiconductor slab on which layers of alternate conductivity have already been formed. The doping impurities which form these layers tend to migrate through the semiconductor material when it is heated.
Chemical vapour deposition is discussed in "On the enhancement of silicon chemical vapour deposition rates at low temperatures" (CHIN-AN-CHANG, J. Electrochem. Soc. Solid-state Science and Technology, vol 123, No. 8, August 1976, p 1245). This paper indicates that the rate at which doped silicon is deposited can be increased by electrically biasing the substrate, but the paper is not concerned with attempting to avoid heating of the substrate altogether.
A third known method consists in introducing a gas such as silane (SiH.sub.4) continuously into a chamber and decomposing it by virtue of the electronic shock effect produced in a plasma by a high-frequency electric field. This method is generally satisfactory, and the rate of deposition can be increased, if necessary, by increasing the power input to the electric field and/or the partial pressure of the silane. When high concentrations of doping impurities such as arsenic are required, by mixing arsine (AsH.sub.3) with the silane, for example, the rate of deposition reaches a "saturation" value, and cannot be further increased by increasing the power input to the high-frequency field.
German published patent application (Deutsche Offenlegungschrift) N.degree. 2,312,774 (VEB Elektromat) proposes a method of depositing a thin film by plasma decomposition in which the substrate is electrically biased with respect to the plasma. The purpose of this is to avoid untimely chemical reactions within the plasma which can lead to a deposit which has no mechanical cohesion. The biasing is provided by a DC generator connected between the substrate and a conductive probe inserted in the plasma. Such biasing is not applicable when the deposited material is insulative since there is a tendency for an oposing charge to build up on the deposit. It does not allow a satisfactory deposition rate to be obtained when the material to be deposited is a poor conductor, e.g. a semiconductor.
Preferred embodiments of the present invention provide a method for depositing thin layers of materials by decomposing a gas to yield a plasma which gives high rates of deposition even when the deposited material is a semiconductor including a high proportion of semiconductor doping impurities.